An example of a conventional type fluid pressure control device which controls the output pressure to the brake cylinder of an air brake device disposed on a railway vehicle is one which uses a 3-position solenoid valve which is controlled to switch between three positions: a supply position in which the output pressure will be increased, an overlapping position in which the output pressure is cut off and maintained at a constant value, and an exhaust position in which the output pressure will be reduced. One such conventional type fluid pressure control device can be seen, for example, in Japanese Laid-Open Utility Model H 7-31020).
This fluid pressure control device can generate an output pressure corresponding to a pressure command signal by outputting drive command values (coil current values) of 3 stages in order to control the 3-position solenoid valve in the exhaust position, supply position or overlapping position upon receiving a pressure command signal equivalent to a brake command. In the case where different drive command values are needed when the 3-position solenoid valve moves from the exhaust position to the overlapping position, and when it moves from the supply position to the overlapping position, there are instances where drive command values corresponding to the overlapping position are set in 2 stages. In this case, drive command values of 4 stages can be input to the 3-position solenoid valve.
However, the above-mentioned fluid pressure control device of the prior art has experienced problems in that, since the switching of the 3-position solenoid valve is normally controlled by 3-stage or 4-stage drive command values, overshoot and undershoot will inevitably occur when the output pressure to the brake cylinder is made to coincide with the pressure command signal. Furthermore, a relatively long time is needed before the output pressure converges with the pressure command signal, and since the 3-position solenoid valve switches frequently, the life of the 3-position solenoid valve is shortened.
Thus, there have been ideas advanced to prevent this undesirable overshoot and undershoot by comparing the output pressure of the 3-position solenoid valve with the pressure command signal and outputting any desired drive command value from the afore-mentioned supply position to the exhaust position, in the direction in which the output pressure and the pressure command signal will coincide, thereby controlling the degree of valve opening of the 3-position solenoid valve.
The aforementioned 3-position solenoid valve has problems in that factors such as variations in the spring force of the return spring, fluctuations in the sliding resistance to sliding parts due to the adhesiveness of the grease, and variations in the attraction characteristics of the coil mean that the degree of valve opening differs a little with each 3-position solenoid valve. It has likewise been observed that during operation even with the same 3-position solenoid valve, the same degree of valve opening will not necessarily be achieved, even with the same pressure command signal, due to changes encountered throughout the year.
In particular, therefore, some problems will likely result in those instances in which, when a drive command value close to the drive command value for the overlapping position is output to the 3-position solenoid valve, the degree of valve opening is even less than required because of the aforementioned variations and the like, and a long time is required before the output pressure corresponds with the pressure command signal, or it settles while there is still a difference between the output pressure and the pressure command signal.